Model tests on deep soft ground improvement of existing sand-filled subgrade with squeezed branch piles

There are limitations from the pavement structure layer and construction site space during the deep soft soil reinforcement of the existing sand-filled subgrade in expansion of expressways. Firstly, a new method using the squeezed branch pile (SBP) with one single plate for treatment of the existing soft foundation is proposed. In this method, a plate is set on the top of the piler to achieve the effects similar to a pile cap or support plate without breaking the road structure layer. Secondly, a physical model test on the pile group foundation is performed, and a pile-soil unit is simulated to study the bearing behavior of the SBP. By analyzing the mechanical and deformation indices of the pile, plate and soil, the bearing performance and soil-deformation characteristics of the new reinforcement method are studied. Finally, a simplified two-dimensional model for the reinforcement method with SBPs for the existing sand-filled embankment is derived by considering the soil parameters and pile sizes. The results indicate that the axial force of piles and soil pressure vary with time under constant loads due to the compressibility of soft soil and the properties of loose sand. The total axial force at the top of the pile is approximately positively correlated with the magnitude of the constant loads linearly. At the same time, the larger the diameter of the support plate, the greater the total axial force at the top of the pile and the higher the proportion of pile-bearing loads. It is found that the pile-bearing ratio of the reinforcement method with SBPs is about 70%. In addition, the proposed two-dimensional simplified model for parameter analysis shows that the minimum pile spacing in the sand-filled subgrade is related to the overlying load, internal friction angle of sand-filled soil, strength index of deep soft soil and geometric parameters of support piles.